Bacillus thuringiensis (B.t.) is a soil-borne bacterium that produces pesticidal crystal proteins known as delta endotoxins or Cry proteins. Cry proteins are oral intoxicants that function by acting on midgut cells of susceptible insects. Some Cry toxins have been shown to have activity against nematodes. An extensive list of delta endotoxins is maintained and regularly updated at the website lifesci.sussex.ac.uk/home/Neil_Crickmore/Bt/intro.html.
Western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is an economically important corn pest that causes an estimated $1 billion revenue loss each year in North America due to crop yield loss and expenditures for insect management (Metcalf, 1986). WCR management practices include crop rotation with soybeans, chemical insecticides and, more recently, transgenic crops expressing B.t. Cry proteins. However, to date only a few examples of B.t. Cry proteins provide commercial levels of efficacy against WCR, including Cry34Ab1/Cry35Ab1 (Ellis et al., 2002), modified Cry3Aa1 (Walters et al., 2008) and modified Cry3Bb1 (Vaughn et al., 2005). These B.t. proteins are highly effective at preventing WCR corn root damage when produced in the roots of transgenic corn (Moellenbeck et al., 2001, Vaughn et al., 2005, U.S. Pat. No. 7,361,813).
Despite the success of WCR-resistant transgenic corn, several factors create the need to discover and develop new Cry proteins to control WCR. First, although production of the currently-deployed Cry proteins in transgenic corn plants provides robust protection against WCR root damage, thereby protecting grain yield, some WCR adults emerge in artificial infestation trials, indicating less than complete larval insect control. Second, development of resistant insect populations threatens the long-term durability of Cry proteins in rootworm control. Lepidopteran insects resistant to Cry proteins have developed in the field for Plutella xylostella (Tabashnik, 1994), Trichoplusia ni (Janmaat and Myers, 2003, 2005), and Helicoverpa zeae (Tabashnik et al., 2008). Insect resistance to B.t. Cry proteins can develop through several mechanisms (Heckel et al., (2007), Pigott and Ellar, 2007). Multiple receptor protein classes for Cry proteins have been identified within insects, and multiple examples exist within each receptor class. Resistance to a particular Cry protein may develop, for example, by means of a mutation within the toxin-binding portion of a cadherin domain of a receptor protein. A further means of resistance may be mediated through a protoxin-processing protease. Resistance to Cry toxins in species of Lepidoptera has a complex genetic basis, with at least four distinct, major resistance genes. Similarly, multiple genes are predicted to control resistance to Cry toxins in species of Coleoptera. Development of new high potency Cry proteins will provide additional tools for WCR management. Cry proteins with different modes of action can be produced in combination in transgenic corn to prevent the development WCR insect resistance and protect the long term utility of B.t. technology for rootworm control.